This application is based on application No. H9-9276 filed in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a resin-encapsulated semiconductor apparatus having a semiconductor device with a ferroelectric film, and a process for its fabrication.
2. Description of the Related Art
In recent years, non-volatile or large-capacity semiconductor memory devices having thin films of ferroelectric substances (dielectric materials having a high dielectric constant, or substances having a perovskite crystalline structure) have been proposed. Ferroelectric films have features such as self polarization and high dielectric constant characteristics. Hence, the ferroelectric films have hysteresis characteristics between polarization and electric fields of ferroelectric substances, and their utilization enables materialization of non-volatile memories. Also, the ferroelectric films have such a larger dielectric constant than silicon oxide films that memory cells can be made to have a smaller area when the ferroelectric films are used as capacitive insulation films, to enable materialization of large-capacitance highly integrated RAMs (random access memories).
The ferroelectric films are comprised of a sintered body of a metal oxide, and contain much oxygen which is rich in reactivity. When capacitors are formed by using such ferroelectric films in the capacitive insulation films, it is indispensable, in the upper and lower electrodes of the capacitive insulation films, to use a substance which is stable to oxidation reaction, as exemplified by an alloy chiefly composed of platinum.
After capacitors, interlayer insulation films and so forth have been formed, passivation films are formed on the outermost surfaces of the devices. Silicon nitride or silicon oxide is used in the interlayer insulation films and passivation films, which are usually formed by CVD (chemical vapor deposition) and hence hydrogen is often incorporated in the films.
When semiconductor apparatuses making use of such ferroelectric films are used in electronic equipment for public use, they are required to be inexpensive resin-encapsulated semiconductor apparatuses having good mass productivity. In particular, ferroelectric non-volatile memories are greatly needed for portable equipment as memories substituting flash memories, because of their properties such as low power, low voltage, and non-volatility making refresh operation unnecessary, and the resin-encapsulated semiconductor apparatuses are also desired in order to provide thin type packages.
At present, however, devices that utilize the ferroelectric films as capacitive insulation films are chiefly held by ceramic-encapsulated products, and almost no resin-encapsulated products are available. Devices with a large capacity are also not yet developed. This is because the polarization characteristics of ferroelectric films deteriorate as a result of heat treatment.
Capacitors having ferroelectric films are known to undergo deterioration of polarization characteristics upon their annealing in an atmosphere of hydrogen (Lecture Collections in ""96 Ferroelectric Film Memory Technique Forum, published by K.K. Science Forum, Inc. Page 4-4). This deterioration is presumed to be caused by the platinum of upper and lower electrodes which reacts with hydrogen to act as a reducing catalyst to reduce the ferroelectric film. In particular, in the case of large-capacity highly integrated devices, the ferroelectric films are fine in size, and hence this deterioration of the characteristics of the capacitors is forecasted to greatly affect the characteristics of the overall devices.
In the resin-encapsulating of semiconductor devices by transfer molding, encapsulant resins containing fillers (usually silica) are used. The fillers contained in encapsulant resins, however, have such hard particles that the fillers may damage the device surfaces when encapsulated. Moreover, since ferroelectric materials exhibit piezoelectricity, the characteristics of ferroelectric films ma y change upon application of a pressure to the ferroelectric film inside the devices when encapsulated. In the fabrication of DRAMs (dynamic random access memories), xcex1-rays are emitted from radioactive components contained in the fillers, to cause memory soft errors in some cases. Accordingly, in order to prevent the device surfaces from being damaged by the fillers, to prevent application of pressure to the ferroelectric films and to screen xcex1-rays being emitted from the fillers, protective films comprised of polyimide must be previously formed on the device surfaces. Such surface-protective polyimide films are formed by heat-curing polyimide precursor composition films usually at a temperature of about 350 to 450xc2x0 C. When such a polyimide precursor is heat-cured, the hydrogen contained in the passivation films or interlayer insulation films may diffuse to cause a deterioration of polarization characteristics of the ferroelectric films. Thus, no resin-encapsulated products of devices in which thermoplastic resins are used as surface-protective films are known at present.
An object of the present invention is to provide a resin-encapsulated semiconductor apparatus having a ferroelectric film with good polarization characteristics and having a high reliability, and a process for its fabrication.
Studies made on conditions under which ferroelectric films cause the deterioration of polarization characteristics have revealed that the deterioration occurs when heated at above 300xc2x0 C. The present inventors thought that the surface-protective polyimide films could be heat-cured at below 300xc2x0 C. However, when conventional polyimide precursors are used, which cure at such a low temperature, the resultant resin-encapsulated semiconductor apparatuses had a problem in their solder reflow resistance.
At present, as methods for packaging resin-encapsulated semiconductor apparatuses on printed-wiring substrates, face-down mounting is prevalent. The face-down mounting employs a solder reflowing method, in which leads of a semiconductor device and wiring of a printed-wiring substrate are provisionally joined with a cream solder followed by heating of the entire semiconductor device and substrate to solder them. As methods sorted according to how heat is applied, infrared reflowing and vapor phase reflowing are known, the former being a method utilizing infrared radiated heat and the latter being a method utilizing condensation heat of fluorinated inert liquid.
As encapsulant resin, epoxy resin is usually used. This epoxy resin always absorbs moisture in an ordinary environment. At the time of solder reflow soldering, resin-encapsulated semiconductor apparatuses are exposed to high temperatures of from 215 to 260xc2x0 C. Hence, when the resin-encapsulated semiconductor apparatuses are packaged on the substrate by reflow soldering, the abrupt evaporation of water causes cracks in the encapsulant resin to bring about a serious problem in view of the reliability of semiconductor devices. Accordingly, in the past, various improvements have been made from the viewpoint of making the encapsulant resin have a lower moisture absorption and have a high adhesion performance (Thermosetting Resins, Vol. 13, No. 4, published 1992, page 37, right column, lines 8-23).
The present inventors have examined resin cracks produced in conventional resin-encapsulated semiconductor devices, and have found that peeling occurs at the interface between the device surface-protective polyimide film and the encapsulant resin, and that this is the starting point of causing cracks in the encapsulant resin. They have also found that this peeling is influenced by physical properties of surface-protective films, in particular, glass transition temperature and Young""s modulus.
Now, as a result of further detailed studies, it has been found that ferroelectric films may cause less deterioration of polarization characteristics when the device surface-protective polyimide films are formed by heat treatment in the temperature range of from 230xc2x0 C. to 300xc2x0 C. It has been also found that, when the polyimide formed at such heat treatment temperature has a glass transition temperature of from 240xc2x0 C. to 400xc2x0 C. and a Young""s modulus of from 2,600 MPa to 6 GPa, the resin-encapsulated semiconductor apparatus has a superior solder reflow resistance and no peeling may occur at the interface between the device surface-protective polyimide film and the encapsulant resin, promising high reliability.
Based on these new findings, the present invention provides a resin-encapsulated semiconductor apparatus comprising a semiconductor device having a ferroelectric film and a surface-protective film, and a encapsulant member comprising a resin, the surface-protective film being formed of a polyimide. The present invention has first made it possible to materialize such a device for the first time.
The present invention also provides a process for fabricating a resin-encapsulated semiconductor apparatus, the process comprising the steps of;
forming a polyimide precursor composition film on the surface of a semiconductor device having a ferroelectric film;
heat-curing the polyimide precursor composition film to form a surface-protective film formed of a polyimide; and
encapsulating with a encapsulant resin the semiconductor device on which the surface-protective film has been formed.
The polyimide used in the present invention as a material for the surface-protective film may preferably have a glass transition temperature of from 240xc2x0 C. to 400xc2x0 C. and a Young""s modulus of from 2,600 MPa to 6 GPa. Use of such a polyimide makes it possible to obtain a semiconductor device having a high reliability, without causing any cracks even by reflow soldering. The polyimide precursor composition film may preferably be heat-cured at a temperature of from 230xc2x0 C. to 300xc2x0 C., but may be done at a temperature higher than 300xc2x0 C. so long as the heat treatment is carried out at 350xc2x0 C. or below for a short time (usually within 4 minutes, depending on the heat resistance of semiconductor devices) and also the polyimide film thus formed has a Young""s modulus of 3,500 MPa or above and a glass transition temperature of 260xc2x0 C. or above, thus the objects of the present invention can be achieved without causing any deterioration of polarization characteristics of the ferroelectric film.
Incidentally, the fabrication process of the present invention may also be applied to resin-encapsulated laminates in which polyimide films are used for purposes other than surface-protective films, e.g., insulating films.